This invention relates to a roller made of synthetic resin used in a belt-conveyor and, more particularly, to an improved roller made of synthetic resin, in which a ball-bearing assembly (hereinafter simply called bearing) may not be affected by the variation of clamping force acted thereon through expansion and contraction of the synthetic resin, which is very unfavorable to the bearing function in prior art rollers. It is an object of the present invention to obtain a carrier roller of low noise, low power consumption, and particularly long life by employing an insert-injection molding with a material which is suited for the molding process, inexpensive, and light in weight.
As materials for belt-conveyor carrier rollers, metals, rubbers, or synthetic resins have been conventionally utilized, in general, although belt-conveyors used for construction work or civil engineering mostly metallic rollers. This can chiefly be attributed to higher strength, less wearing, and greater durability of the metallic materials. Metal rollers are however not free from some disadvantages, such as noises, heavy weight of the rotatable portion, which causes an increase of rolling resistance accompanied by the running resistance of the entire conveyor system. It consequently brings about an increase in the resultant power consumed, equipment cost, and also running cost. Attempts at eliminating these disadvantages have turned conveyor makers to a material change, from metals to synthetic resins or rubbers, with some success indeed, such as noise decrease and weight decrease of the rolling parts. Irrespective of these merits and further secondary favorable effects of decrease of running resistance or power saving, some dissatisfactions have still been left unsolved in strength and, durability as well as in maintenance. Particularly, synthetic resins produced by the injection method are subjected to so-called post-shrinkage during a fairly long time after molding, in addition to the well-known molding shrinkage at the time of the injection process. In a roller, wherein a bearing assembly is fitted thereinto, fluctuation of the bearing-sustaining power owing to the thermal expansion or contraction of the synthetic resin, along with the abovementioned post-shrinkage, will cause slipping or skidding of the bearing sustaining portion, even when the bearing is forcedly fitted into the roller after it has been fully machined. Once this skidding occurs, smooth rotation of the roller is hindered to extremely shorten the life of the bearing.